LiF doped Mg-Al-Spinel

The Sintermechanism of LiF-doped Mg-Al-Spinel

Mathis M. Müller, Hans-Joachim Kleebe

In an actual study the synthesis of transparent and polycrystalline spinel ceramics is determined by means of Transmission Electron Microscopy.

Caused by an elevated hardness and a wide transmission window for IR radiation, those ceramics provide a wide range of applications e.g. as armor material or IR translucent windows. To reach the transparency an absolutely dense ceramic without any porosity is required. In former studies it was shown, that this requirement can be reached by the addition of LiF as a sintering aid. To understand the densification mechanism of magnesium aluminate spinel with addition of LiF, the system will be investigated systematically.

In former studies, various mechanisms considering the effect of the transient LiF phase were proposed such as an early stage densification at lower temperatures by forming a wetting liquid phase affecting grain growth, an enhancement of diffusion by forming oxygen vacancies due to the incorporation of LiF into the spinel crystal lattice, or the interaction of LiF with impurities, whereby LiF acts as a cleanser. However, there is no consistent picture of the sintering mechanism, promoted by LiF addition, as yet. A critical question is the continuance of the transient LiF phase or its alteration products during sintering at elevated temperatures, knowing that a transient LiF phase is not detectable in transparent spinel sintered above 1500 °C.

The aim of transparent spinel ceramics was phenomenologically reached using HIP-sinter techniques, whereby the sinter mechanism behind are not fully understood.

The overall aim of this investigation is (a) to verify the effective densification mechanism upon LiF addition to improve the actual sinter techniques and (b) based on the findings to transfer this knowledge to pressure less densification techniques.